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Chapter 6
Volcanic Eruption
Volcano
Year
Vol. Ejecta (km3)
St. Helens
St. Helens
St. Helens
Vesuvius
Pinatubo
Krakatoa
Katmai
Tambora
Mazama
Yellowstone
1500
1
1980
1
1900 BCE
4
79 CE
9
1991
8.4-10.4
1883
20
1912
30
1815
80
4850 BCE 150 (Crater Lake OR)
600,000 BCE
280-2,500
•
Cities of Pompeii and Herculaneum buried by massive
eruption of Mt. Vesuvius
•
Similar to 1991 eruption of Mt. Pinatubo in
Philippines
•
Clouds of hot gas (850oC, iron welding temperature)
and ash enveloped city, burying many in pumice or
killed by pyroclastic flows
• The last phase of eruption blew ash up to 32 km in
the atmosphere, buried Pompeii under additional 2 m
of debris
•
Lahars (volcanic mudflow) & pyroclastic flows
buried city of Herculaneum up to 20 m deep
• 4,000 people killed
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• Vesuvius continued to erupt about every 100 years
until about 1037 A. D. when it became dormant for
600 years.
• In 1631 it erupted killing an additional 3,500
inhabitants. Again in 1944, but today about 2
million people live in at the foot of Vesuvius in
Naples and Pompeii.
–
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Understanding Volcanic Eruptions
•
View in context of plate tectonics
•
Variations in magma:
– chemical composition and gas content
– ability to flow,
– volume
– determines whether eruptions are peaceful or
explosive
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Most volcanism is associated with plate boundaries
– 80% at spreading centers
– About 10% at subduction zones
Remaining 10% of volcanism occurs above hot spots
Chemical and Mineral Compoisition of Magmas
 Eight elements make up more than 98% of Earth’s
crust
 Oxygen and silicon are by far most abundant

Typically join up as SiO4 tetrahedron, that ties
up with positively charge atoms to form minerals
 Mineral formation in magma: crystallization
–
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Order of crystallization of different minerals in
magma can be determined starting with high
temperature.
– Iron and magnesium link with aluminum and SiO4 to
form olivine, pyroxene, amphibole and biotite
families
– Calcium combines with aluminum and SiO4 until
calcium replaced by sodium, to form plagioclase
feldspar family;
• calcium and sodium are later replaced by potassium,
to form potassium feldspar and muscovite families;
finally only Si and O remain, forming quartz at the
lowest temperature.
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•
•
Elements combine to form minerals
Minerals combine to form rocks
Different compositions of magma result in different
igneous rocks
•
If magma cools slowly and solidifies beneath
surface  plutonic rocks (Granite and Gabbro)
If magma erupts and cools quickly at surface 
volcanic rocks (Rhyolite and Basalt)
Crustal Elements (%W)
Continental
Crust
SiO2
60.2%
Al2O3
15.2
Fe2O3
2.5
Oceanic
Crust
48.7%
16.5
2.3
FeO
CaO
3.8
5.5
6.2
12.3
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•
•
Viscosity: internal resistance to flow
– Lower viscosity  more fluid behavior
• Water, melted ice-cream
– Higher viscosity  thicker
• Honey, toothpaste
Viscosity determined by:
– Higher temperature  lower viscosity
– More silicon and oxygen tetrahedra  higher
viscosity
• More mineral crystals  higher viscosity
•
Consider three types of magma and rocks they form:
basalt, andesite and rhyolite
– Basalt has highest temperatures and lowest SiO2
content, so lowest viscosity (fluid flow)
– Low water content
– Peaceful, Safe Eruptions
– Rhyolite has lowest temperatures and highest SiO2
content, so highest viscosity (does not flow)
– High water content (steam cannot escape magma)
– Violent, Dangerous Eruptions
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– Basalt at spreading centers (e.g. Rifts)
– 80% of magma that reaches Earth’s surface, at
spreading centers
– It forms from melting of the mantle
– Melted mantle at subduction zones
– rises through continental crust before reaching
the surface
– incorporating continental high SiO2 rock as it
rises
– becomes andesitic or rhyolitic in composition
before it erupts
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Spreading centers have abundant volcanism because:
– Spreading centers are above hot asthenosphere
–
Asthenosphere has low SiO2
– Plates pull apart so asthenosphere rises and
melts under low pressure, changing to hightemperature
– Low SiO2, low volatile, low viscosity basaltic magma
allows easy escape of gases  peaceful eruptions
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Subduction zones have violent eruptions because:
– Magma is generated by partial melting of the
subducting plate with water in it
– Melts overlying crust to produce magmas of
variable composition
– Magma temperature decreases
– SiO2, water content and viscosity increase 
violent eruptions
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Three things will cause rock to melt:
– Lowering pressure
– Raising temperature
– Increasing water content
• Lowering pressure is most common way to melt rock
A decrease in pressure Liquifies Quartz if the
temperature is greater than 1800 deg C.
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• High pressure of Magma - gases stay dissolved
• Magma rises - pressure decreases
Gas bubbles form
Magma propelled up
• Bubbles expand
fragmenting magma
explodes as a gas jet
•
•
•
Geyser: eruption of water superheated by magma
•
Water can be heated to higher than boiling
temperature (100oC) if it is under pressure
– When superheated water reaches point of lower
pressure, it flashes to steam violently, and
Can only exist in areas of high heat flow
underground
erupts out of the ground
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Nonexplosive eruptions:
– Pahoehoe: smooth ropy rock from highly liquid
lava
– Aa: rough blocky rock from more viscous lava
– Pillow lava: rounded blobs of rock from lava
flowing into water
•
Explosive eruptions:
– Pyroclastic debris: broken up fragments of magma
and rock from violent gaseous explosions,
classified by size
– May be deposited as:
• Air-fall layers (settled from ash cloud)
• High-speed, gas-charged flow that surges over
surface (pyroclastic flow)
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Very quick cooling:
– Obsidian: volcanic glass forms when magma cools
very fast
Pumice: porous rock from cooled froth of magma and
bubbles
Volcanic Explosivity Index
– Provides a means of evaluating eruptions
according to volume of material erupted, height
of eruption column and duration of major eruptive
blast  scale from 0 to 8
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Eruption Styles
– Many different landforms result from volcanic
eruptions or cooling of magma beneath the surface
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•
•
Non-explosive:
–
–
Icelandic
Hawaiian
Somewhat explosive:
–
Strombolian
Explosive:
–
–
Vulcanian
Plinian
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Viscosity may be low or high
– Controls whether magma flows easily or piles up
•
Volatile abundance may be low, medium or high
– May ooze out harmlessly or explode
•
Volume may be small, medium or large
– Greater volume  more intense eruption
– By mixing different values for the three V’s, can
define volcanic landforms and forecast eruptive
styles for each
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Shield Volcanoes: Low Viscosity, Low Volatiles, Large Volume
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Basaltic lava with low viscosity and low volatiles
flows to form gently dipping, thin layers
•
Thousands of layers on top of each other form very
broad, gently sloping volcano like Mauna Loa in
Hawaii
 Great width compared to height
 KILAUEA ERUPTION HAS CONTINUED SINCE 1983
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• “Curtain of fire”: lines of lava fountains up to
300 m high
• Low cone with high fountains of magma
– Floods of lava spill out and flow in rivers down
slope
– Eruptions last days or years
– Eruptions are usually not life-threatening but
destroy buildings and roads
– Hawaiian volcanoes include Haleakala on Maui,
five volcanoes of island of Hawaii and subsea
Loihi (969 m below sea level)
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Rare Hawaiian killer pyroclastic events
– King Keoua’s army passing through Kilauea area
was stopped by eruptions and split into three
groups to escape area
– Base surge overtook middle group, killing all 80
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Icelandic-Type Eruptions
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•
Most peaceful type of eruption
•
Low-viscosity, low-volatile lava flows almost like
water
– Build up volcanic plateaus (even flatter than
shield volcanoes) of nearly horizontal basalt
layers
Fissure eruptions:
– Lava pours out of linear vents or long fractures
up to 25 km long
– “Curtain of fire” effect
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Flood Basalts
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•
•
Largest volcanic events known on Earth
•
Can indirectly have global effects as huge amounts
of gases (including CO2 and SO2) are released into
the atmosphere
•
Immense amounts of basalt erupted
Geologically short time (1 to 3 Million years)
– Different from hot spots that last hundreds of
millions of years and subduction zones which last
tens of millions of years
Some flood basalts coincide with mass extinctions:
– Siberia (250 million years ago): 3 million km3 of
basalt
– India (65 million years ago): 1.5 million km3 of
basalt
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Scoria Cones: Medium Viscosity, Medium Volatiles, Small
Volume
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Low conical hills (also known as cinder cones) of
basaltic to andesitic pyroclastic debris built up
at volcanic vent
•
Can have summit crater with lava lake during
eruption
•
Form during eruptions lasting hours to several
years
•
After eruptions cease, scoria cones usually have
central vent of hard lava with softer layers of
pyroclastic debris
– Erosion may eventually leave only central vent
standing above countryside as volcanic neck
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Strombolian Type Eruptions
•
•
Scoria cones usually built by Strombolian eruptions
Named for Stromboli volcano in Italy, erupting
almost daily for millenia (tourist attraction)
– Central lava lake with thin crust that breaks
easily to allow occasional frequent eruptive
blasts of lava and pyroclastic debris
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Stratovolcanoes (also known as composite volcanoes)
– Steep-sided, symmetrical volcanic peaks
– Composed of alternating layers of pyroclastic
debris and andesitic to rhyolitic lava flows
– Eruptive styles from Vulcanian to Plinian
Vulcanian Type Eruptions
•
Alternate between highly viscous lava flows and
pyroclastic eruptions
•
Common in early phase of eruptive sequence before
larger eruptions (‘clearing throat’)
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Plinian Type Eruptions
• Named for Pliny the Younger (descriptions of 79
C.E. eruption of Mt. Vesuvius)
• Occur after ‘throat is clear’, commonly final
eruptive phase
• Gas-powered vertical columns of pyroclastic debris
up to 50 km into the atmosphere
– Can create ‘volcano weather’, when eruption column
causes rain mixing with ash on volcano’s slopes
and creating mudflows (lahars) that can be
devastating
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Lava Domes: High Viscosity, Low Volatiles, Small Volume
• Form when high-viscosity magma at vent of volcano
cools quickly  hardened plug
– Gases accumulate at top of magma chamber and
power Vulcanian and Plinian blasts until most
volatiles have escaped
– Remaining magma is low-volatile, high-viscosity
paste
– Oozes to vent and cools quickly in place, forming
plug
– If plug is big enough, can fall in landslide,
triggering small but violent eruption
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Volcanic Eruption
Typical Eruption Sequence
•
•
Gas-rich materials shoot out first as Vulcanian
blast, followed by longer Plinian eruption
After gas depleted, high-viscosity magma builds
lava dome over long period
– Vulcanian precursor  Plinian main event  lava
dome conclusion
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Calderas: large volcanic depressions (larger than
crater) formed by roof collapse into partially
emptied magma reservoirs
– Eruptions are largest of violent, explosive
volcanic styles (ultra-Plinian)
•
Form at summits of shield volcanoes or
stratovolcanoes, or as giant continental calderas
(Yellowstone or Long Valley, California)
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Crater Lake
• Formed about 7,600 years ago from Cascade Range
stratovolcano Mt. Mazama
• Major eruptive sequence of pyroclastic flows and
Plinian columns emitted ash layer recognizable
across North America
• Large enough volume of magma erupted to leave void
beneath surface  mountain collapsed into void
leaving caldera crater at surface that filled with
water to form Crater Lake
– 1,000 year old successor volcanic cone Wizard
Island
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Krakatau, Indonesia, 1883
•
Part of volcanic arc above subduction zone between
Sumatra & Java
•
After earlier collapse, Krakatau built up during
17th century
– Quiet for two centuries then resumed activity in
1883
– Moderate Vulcanian eruptions from dozen vents
– Led up to enormous Plinian blasts and eruptions
80 km high and audible 5,000 km away
– Blew out 450 m high islands into 275 m deep hole
– Triggered tsunami 35 m high killing 36,000 people
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• Subduction of Mediterranean plate beneath Europe 
many volcanoes including stratovolcano Santorini
• Around 1628 B.C.E. Santorini underwent series of
eruptions:
– 6 m thick layer of air-settled pumice
– Several meter thick deposits from hot water when
seawater reached magma chamber  steam blasts
– Up to 56 m thick jumbled mass of ash, pumice,
rock fragments from collapse of volcanic cones
– Layers of ash and rock fragments from magma body
degassing
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• Effects on local Minoan culture:
– Akrotiri had three-story houses, sewers, ceramics
and jewelry, trade with surrounding cultures
– Destruction of part of Minoan civilization made
great impact  story of disappearance of island
empire of Atlantis made be rooted in this event
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Hot Spots
• Shallow hot rock masses/magmas or plumes of slowly
rising mantle rock operating for about 100 million
years
• Used as reference points for plate movement because
almost stationary, while plates move above them
• 122 active in last 10 million years, largest number
under Africa (stationary plate concentrates mantle
heat)
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• Oceanic hot spots:
Peaceful eruptions build shield volcanoes (Hawaii)
• Spreading center hot spots:
– Much greater volume of basaltic magma, peaceful
(Iceland)
• Continental hot spots:
– Incredibly explosive eruptions as rising magma
absorbs continental rock, form calderas
(Yellowstone)
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–
–
–
–
Three calderas in U.S. known to have erupted in
last million years:
– Valles caldera in New Mexico, about 1 million
years ago, in Rio Grande rift
– Long Valley, California, about 760,000 years ago,
edge of Basin and Range
– Yellowstone, Wyoming, about 600,000 years ago,
above a hot spot
Large volumes of basaltic magma intrude to shallow
depths
Melts surrounding continental rock
Plinian Eruption
Too much magma to all go airborne
Creates pyroclastic flows
– Magma body sinks
– Collapse of surface
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Yellowstone National Park
• Resurgent caldera above hot spot below North
America, body of rhyolitic magma 5 to 10 km deep
• North American plate movement is recorded by trail
of volcanism to the southwest
• Three recent catastrophic (ultra-Plinian)
eruptions:
– 2 million years ago, 2,500 km3
– 1.3 million years ago, 280 km3
– 0.6 million years ago, 1,000 km3,
created caldera 75 km by 45 km,
– covering surrounding 30,000 km2
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Mount Etna - Schizophrenic
• Largest volcano in Europe
•
• Usually Strombolian
•
• Sometimes produces Lava Fountains (Like
Hawaii)
•
– Occasionally Vulcanian